This invention relates to a magnetic refrigerator using a working material which radiates heat when a magnetic field is applied thereto and absorbs heat when the magnetic field is removed.
Magnetic refrigerators are based on the well-known phenomenon that materials, such as gadolinium-gallium-garnet (GGG; Gd.sub.3 Ga.sub.5 O.sub.12) or other rare-earth element compounds or alloys including an alloy of erbium and nickel, radiate heat when a magnetic field is applied to them, and absorb heat when the magnetic field is removed.
These refrigerators have been developed in order to cool helium gas to a very low temperature. Liquid helium at a very low temperature is necessary for cooling superconductive magnets used in nuclear fusion research and for linear motor cars and computers using Josephson elements. Thus, the damand for high-performance refrigerators is increasing in these fields.
A prior art magnetic refrigerator is shown in FIG. 1. In this refrigerator, two superconductive coils 3 and 4 are fixed at a space in a container 2 containing liquid helium 1 at a temperature of 4.2.degree. K. A cylinder 12 is coaxially fixed between the coils 3 and 4. A piston 5 is slidably passed through the cylinder 12. Both extended portions of the piston 5 can individually penetrate the superconductive coils 3 and 4. The piston 5 includes lower, intermediate, and upper portions 5a, 5b and 5c spaced axially. The lower and intermediate portions 5a and 5b are coupled by a working material 7 between them, while the intermediate and upper portions 5b and 5c are connected by another working material 6 between them. The working materials 6 and 7 may be formed of, e.g., gadolinium-gallium-garnet. A support tube 13 is coaxially attached to the outer peripheral surface of the cylinder 12, and a cylindrical adiabatic member 11 is coaxially fixed to the outer peripheral surface of the support tube 13. An annular opening is bored through the central portions of the peripheral walls of the cylinder 12 and the support tube 13. Thus defined is a helium bath 10 which is enclosed with the adiabatic member 11 and penetrated by the piston 5. A material to be cooled (helium in the case) is sealed in the helium bath 10. This helium is at a temperature of 4.2.degree. K., and is intended to be cooled to a lower temperature of 1.8.degree. K.
The operation of the prior art magnetic refrigerator of the aforementioned construction will now be described.
First, the superconductive coils 3 and 4 are energized to form magnetic fields therein. Then, the piston 5 is reciprocated between an upper position as shown in FIG. 1 and a lower position. When the piston 5 is in the upper position, the first working material 6 is located in the magnetic field produced by the one superconductive coil 3, and radiates heat therefrom. When the piston 5 is then moved downward, the working material 6 leaves the magnetic field, and absorbs heat therein and brought into the helium bath 10. As a result, the helium in the helium bath 10 is cooled by the working material 6. When the piston 5 is in the lower position, the second working material 7 is located in the magnetic field produced by the other superconductive coil 4, and radiates heat therefrom. Then, the piston 5 is moved upward, the working material 7 is brought into the helium bath 10, and the helium is cooled.
In the refrigerator thus constructed, the piston 5 slides in the cylinder 12, so that heat is produced by friction between the piston 5 and the cylinder 12. The helium in the helium bath 10 is heated by this frictional heat to lower the refrigerating efficiency.